This disclosure generally relates to compositions comprising at least one polymer and at least one antistatic agent and more particularly, relates to fibers, films, fabrics, coatings, and molded or blown articles comprising the antistatic polymer compositions. In other aspects, this disclosure also relates to processes for imparting antistatic characteristics to substrates.
Static electricity is generated whenever dissimilar materials move or abrade against another object. In the case of immobile objects, even friction on the surface with ambient air can create static electricity. The charge capacity of a substance, defined as the capacity to generate static electricity, depends on, among others, the condition of its surface, the dielectric constant, the surface resistivity, and the relative humidity. Because charge capacity is directly proportional to the surface resistivity, it follows that a material with higher surface resistivity, or one that is better insulator will tend to generate a greater static charge. Accumulated static charge on an insulating surface can range from a few volts up to several hundred thousand volts. Thus, electrostatic discharge becomes an increasingly worrying issue at higher levels of static charge buildup. High levels of static electricity can cause permanent damage to electronic components that work typically at microvolt levels.
Most of the polymers that are used to make plastics are extremely good insulators, or in other words, they have an extremely low surface conductance, or an extremely high surface resistivity. This property makes polymers useful for fabricating electrical equipment. However, polymers can build large electrical charges that create dirt-attracting forces and naturally seek a conductive discharge path. Moreover, polymers generally have very low surface conductance, thus, the decay or discharge rate lasts a very long time, a time during which the material would retain the charge, and thus attract and retain dirt particles.
Antistatic agents constitute a unique class of polymer additives and provide a measure of safety by preventing any fire, resulting from sparking, caused by an accumulation of static electricity on the surface of an article fabricated of the polymer. They also offer aesthetic values by preventing the accumulation of surface dust on the article. For example, lenses of automotive headlamps are typically made of polymers, such as polycarbonates, which have the desirable combination of heat stability, dimensional stability, transparency, and ductility. In the past, the optics system (also sometimes called “Fresnel”) necessary to properly focus the headlight beam on the road did not have a smooth profile. Consequently, the dust that accumulated on the lens surface, either during the lens molding step, or during the service life of the headlamp, was not conspicuously visible. But with the automotive industry moving towards lenses with a smoother profile, the accumulated dust becomes more easily visible, therefore leading to aesthetics issues. Thus, automotive headlamp manufacturers are looking for alternative materials that have enhanced antistatic properties without, of course, compromising on the other desirable properties the current materials already possess.
Another important area, where mitigation of static charge buildup is critical, is in conveyor belt design. For the most part nowadays, metal conveyor belts have been replaced and are made mostly of plastics and/or synthetic polymeric materials. The replacement of metal with plastic has led to several distinct advantages in conveyor belt technology, such as cleanliness (plastic parts shed fewer particles), reliability (plastic conveyor belts work for very long hours without attention), relatively lower noise (plastic parts naturally damp out clanging and resonant vibration that typically accompany metal based processes), low cost to lifetime ratio (plastic parts undergo much slower mechanical abrasion than metal-based systems), modularity and flexibility, precision due to tight tolerances in the original plastic conveyor components, and automation adaptability made possible by simple retrofit of external systems under electric control.
The advantages of the plastic conveyor belts, outlined above, have served very well in meeting the needs of modern production needs over the past two decades. Then in the 1990's, plastics-based conveyor systems began to be used in hyper-clean environments (Class 100 or higher) essential for manufacture of advanced electronics products and systems. But as product dimensions and tolerances began to approach sub-micron levels, electrostatic discharge, a phenomenon inherent in plastic materials formulated without antistatic agents, posed difficulties to the high technology manufacturer employing plastic conveyor belt components. The buildup of surface charge also results in secondary dirt contamination, which has undesirable consequences, especially for precision, high technology electronic components. Since the conveyor belt functions through a combination of motion and friction, the belts tend to build up large amounts of electrostatic charge on their surface, thus leading to an increased possibility of electrostatic discharge. The damaging consequences of an electrostatic charge on precision electronic equipments have already been described above. It therefore becomes clear that for synthetic polymers to continue to serve the increasingly demanding requirements of the conveyor belt market, more effective plastic materials capable of effective surface charge dissipation are required.
Antistatic agents have generally been applied in one of two ways: externally and internally. Spraying the surface, or dipping the polymeric plastic material in a medium containing the antistatic agent can be used to externally apply the antistatic agents. On the other hand, internally applied antistatic agents are generally added to the polymer before processing. For this reason, internal antistatic agents have to be thermally stable and be able to migrate to the surface during processing to impart the most effective antistatic decay behavior.
There are many antistatic agents having a surface-active component (surfactant-like) within its structure. Internal antistatic agents of the anionic surfactant type are generally difficult to handle because they are inferior in compatibility and uniform dispersibility. Cationic surfactants containing quaternary nitrogen have good antistatic characteristics, but have limited utility. Non-ionic surfactants generally have inferior antistatic characteristics compared to the ionic varieties. Moreover, due to the limited thermal stability of surfactants in general, they are typically not used for processing engineering thermoplastics, such as polycarbonates. Metal salts of organic sulfonic acids have been used as antistatic agents, but they are not thermally stable, and not sufficiently compatible with resins.
It is therefore desirable to identify more effective antistatic agents as additives such that they can be incorporated into polymers without adversely affecting the physical and chemical properties of the resulting polymer compositions. The antistatic additives and compositions thereof described herein are extremely useful for producing articles with outstanding abilities to dissipate static charge buildup, and mitigate or eliminate problems due to dust attraction/repulsion. This in turn leads to enhanced performance, safety, and aesthetic features for these articles.